1. Field of the Invention
This invention relates in general to packoff assemblies for sealing in casing in a well, and in particular to a packoff assembly that is set on wireline.
2. Description of the Prior Art
There are occasions when testing and treating wells, that a packoff assembly, particularly a bridge plug, must be set at different levels. For example, a bridge plug may be set above a lower zone previously tested to isolate the lower zone from a zone above to be tested. If there are a number of zones to be tested in the well, the bridge plug may be reset at various levels.
One method for setting bridge plugs is to lower them on tubing. The bridge plug will be set by manipulating the tubing, typically rotating it. While this is satisfactory, setting the bridge plug initially on tubing is not very accurate. In a deep well, the depth of the bridge plug may be off a considerable amount.
Bridge plugs are also set on wireline. The bridge plug is connected to a wireline tool, lowered to the desired depth, then set. Normally, the bridge plug is set in this manner by sending an electrical charge from the surface to ignite an explosive to cause the setting. The bridge plug can be set at a very accurate depth because the wireline operator can tie in the depth to previous logs of the formation made before running casing.
Some wireline set bridge plugs can be retrieved. They are retrieved by lowering tubing onto the bridge plug and rotating or picking up the tubing to release the bridge plug. These bridge plugs cannot be reset to a different depth. Either a new bridge plug must be lowered from the surface on tubing, or on wireline again.
U.S. Pat. No. 4,593,765 discloses a resettable bridge plug. That bridge plug is initially ran on wireline. It can be released and reset on tubing without pulling the bridge plug to the surface. While the tool shown in U.S. Pat. No. 4,593,765 appears feasible, improvements are desirable.
One danger that is present with a retrievable bridge plug is the possibility of considerably higher pressure below the bridge plug than above. The pressure across the seal elements should equalize before the slips and seal elements are released from the set position. Retrievable bridge plugs do have bypass passages that allow the pressure to equalize. However, these bypass passages normally open simultaneously with the release of tension on the slips. There is a possibility of the bridge plug being blown up the well before the pressure fully equalizes. This could corkscrew the tubing and cause considerable damage.